Permanent rare earth-iron-boron-metal (R—Fe—B-M) magnets are generally produced by powder metallurgical methods. Firstly, an ingot is produced by a casting method. The ingot may be produced by casting the molten alloy into a mold, where it cools comparatively slowly. Alternatively, the ingot may be produced by a rapid solidification method such as strip casting. The solidified ingot is typically given an annealing heat treatment to homogenise the composition.
The ingot may then be given a hydrogenation treatment which is typically used to coarsely pulverise the solidified ingot due to the effects of hydrogen embrittlement of phases within the alloy. The ingot, or resulting coarsely pulverised material, is then further pulverised to produce a powder.
A magnet is produced from the powder by powder metallurgy. The powder is compacted in a magnetic field to form a textured green body which is then given a sintering heat treatment in order to produce a permanent magnet.
It is known that the magnetic properties, in particular the coercive force and the squareness of the J(H) curve, as well as the corrosion resistance and the temperature stability of the sintered magnet depend on the grain size as well as on the composition of the magnet. The composition and grain size of the sintered magnet are, in turn, dependent on the particle size and composition of the powder. R—Fe—B-M powders are, however, rather difficult to manufacture in large quantities and, consequently, the powders and the magnets produced using them are relatively expensive.
It is, therefore, desired to produce high-quality rare earth iron boron (R—Fe—B-M) sintered magnets more cost effectively so as to promote all kinds of applications in which they can be used. It is also desired to improve the corrosion stability and the temperature stability of such magnets.